Ethiopian runner Haile Gebrselassie holds the current world record in the marathon at 2 hours, 3 minutes, and 59 seconds. Here's the question: what's it going to take to shave those last four minutes off for the first two-hour marathon?

A century ago, the world record for the marathon was only a little under three hours, and it would not be until Albert Michelsen in 1925 that the world saw the first marathon run in less than two and a half hours. Since then, the sport has seen constant incremental decreases in the record, and the question now is whether we are reaching the limits of human capabilities. After all, the first marathon run in 2 hours and 10 minutes happened 44 years ago, and we've only been able to shave off six minutes since then.

For his part, Gebrselassie is convinced that a two-hour marathon will happen in the relatively near future:

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"No question. The first sub two-hour marathon will need 20 to 25 years, but it will definitely happen."

His is only one opinion, however, and for every person who shares his optimism there are others who suspect two hours lies beyond human abilities. There's no shame in that, of course - a two-hour marathon, after all, would require the runner to average 13.2 miles per hour for the entire length of the race. American marathon expert Glenn Latimer counts himself among the skeptics:

"Maybe that's because I'm old, but I don't see it happening in a long, long time. You watch these great athletes up close, an athlete as great as Haile Gebrselassie... and you could see the strain, he looks magnificent through 20, 21, 22 miles and then it starts, and then the body starts to break itself down and maintaining pace is hard enough."

This particular achievement is the more extreme equivalent of Roger Bannister's legendary four minute mile, a feat that in its time was also thought to be impossible. Runners, experts on the sport, and even scientists have to figure out just what the upper limits of human athletic ability really are, and there's just no way to know for sure. For his part, Latimer suspects 2 hours 2 minutes will be the upper limit, with no further improvement beyond that.

* Their maximal rate of oxygen consumption, known as VO2 max
* Their running efficiency - how quickly they can cover the ground
* Their endurance capability - what percentage of their VO2 max they can sustain

You figure out the limits of these three elements, you then know the fastest possible marathon time. Of course, the only real way to find that out is to just have people run faster and faster until we discover that it's impossible to do any better.

Based on current rates of improvement in marathon times, University of Montreal Francois Peronnet estimates that 2028 will be the year of the two-hour marathon. Although experts remain divided on whether the feat is actually possible, everyone pretty much agrees that, if any runner is going to do it, he or she will need the perfect combination of factors:

* First, it will need an elite athlete in tip-top condition, probably one from east Africa.
* Second, it will need to be on a fast, flat course such as Berlin, London or Rotterdam. Berlin is known as one of the quickest and has produced four world records in the last 10 years.
* Third, perfect weather conditions. No wind and temperatures of around 10-15C.
* Fourth, decent pace-makers to lead the race and take the elite round at the right speed.
* Finally, money. As the marathon gets closer to the magic mark, race directors will dangle huge financial carrots to incentivise runners to break it. The first person to dip under two hours will run into the record books a very rich person.